Air preheater for furnaces



May 27, 1952 Filed March 20, 194'? C. A. LARSON AIR PREHEATER FOR FURNACES 3 Sheets-Sheet l y 1952 c. A. LARSON 2,598,395

AIR PREHEATER FOR FURNACES Filed March 20, 1947 3 Sheets-Sheet 2 May 27, 1952 c. A. LARSON AIR PREHEATER FOR FURNACE 3 Sheets-Sheet 3 Filed March 20, 1947 Patented May 27, 1952 AIR PREHEATER, FGRFURNACES Charles A; Larson, Fergus -Fall's,"'Minn., 'assignor of one-third to :Leopolcl'Pistner, St. Paul; and one-third to Geo'firey W. Welch, Fergus Falls,

Minn.

'ApplicatlonMarclrzm 1947, Serial No. 736,053

3 Claims.

1 This invention relates to preheatersforsupplying air at'unusually high temperatures for combustion in furnaces and/or'for reducing'the moisture content and facilitating the ignition of "solid sub-bituminous or other low grade fuels.

It is'an object of my invention to provide a highly. eflicient heater of this class having air conduits or tubes which are exposed to intense radiant heat within the combustion chamber of afurnace.

A further object is to provide a heater of this class which is adapted to furnish large amounts of air at'controlled temperatures ranging above 600 degrees F.

Another object is to provide an 'air'heater with means for feeding air 'atroom temperature and pressure above atmospheric to a multiplicity of metallic heater tubes exposed :to direct radiation from burning fuel in the combustion chamber and 1 to protect such tubes fromtheeffect of :excessively high or damaging temperature by locatingthem in close proximity or heat exchange relation to waterwall -tubes within the combustion chamher.

The invention also includes certain other novel 11 illustrate, by way of example and not for the pur- 11:

pose of limitation, a'preferred embodiment of my invention: 2

Figure 1 is afrontelevational view showing my air pre-heater mounted on the front wall of a furnace adapted to burn sub-bituminous fuel;

Fig. 2 is a plan view of the heating apparatus together with a fragmentary portion of the front furnace wall in horizontal section;

Fig. 3 is a fragmentary elevational view showing particularly the arrangement of air tubes and water wall tubes as viewed from the interior of the furnace, a fragmentary portion of the furnace wall being shown;

Fig. 4 is a fragmentary vertical section through the furnace front wall, showing my air heating apparatus partially in side elevation-and partially in vertical section, together with a portion of a chain grate stoker for the furnace;

Fig. 5 is a detail plan view showing three of the hot air discharge nozzles and adjacent portion of the header;

Fig. 6 is a detail side view of one of the nozzles and showing the header in section, and

Fig. 7 is an end view of one of the nozzles.

My improved air heater is shown in connection with a steam generating furnace having a front wall 8 and side walls '9 (Fig. 4) of suitable refractory material. The front wall 8 is formed with a combustion arch ID projectingabove a fuel gate .111 into which fuel 12 may be fed by suitable '2 means, e..g .,,a traveling chain' 'grate stoker indicated diagrammatically at I3 in'Fig's. l'andf l. Combustionof'the fuel takes place in the chamber indicated generally at I4 (Fig. 4).

The. air preheating apparatus comprises a multiplicity of thin walled metallic tubes [5 extending'in parallel relation one to another uponthe inner surface of the wall 8 and severally connected at their upper and lower ends respectively to headers I6 and I! mounted exteriorly 'Offltlie furnace. The tubes [5 may be constructed from high chromium steel or other high temperature resistant alloy. Air at room or atmospheric temperature is supplied to the header [6 by motor driven blowers is connected respectively to branches IQ of an air conduit 20 communicating with the central portion of the header I6. 'Another branch 2| of the conduit 20 extends down- Ward from the branches I9 and is connected to the header I1 at .a central point. The conduit 28 and each of the branches I9 and H are provided with dampers, indicated at 2.2, tope'r'mit selectiverestriction of the flow of air through these conduits.

From the ends of the header I! air conduits 23 extend downward and laterally atanielevation below the grate I3 where theseconduits are connected together by a pipe 24 from which'the hot air-is carried upward througha pipe 25 to an outletheaderZG above the'gra'te l3. From the header 26 a multiplicity of nozzles 21 project horizontally to discharge heated air directly into the body of fuel I2 as it is carried into the combustion chamber. Good distribution of the hot air in the body of fuel is afforded by providing a multiplicity of nozzles 21 which have laterally elongated discharge ends 2Ia.. Heat expansion joints 28 are provided intermediate the ends of the conduits 23 and 2i and these and the other conduits and piping carrying the high temperature air may be encased in suitable heat insulating material.

The portions of the air tubes l5 which are exposed to direct radiation within the combustion chamber l4 extend in parallel relation and closely adjacent to water wall tubes 29 which also extend upon the inner surface of the wall 8. Water is supplied to the lower ends of the tubes 29from a header 30 and the water, as it is heated in the tubes 29, rises and is collected in a header at their upper ends, not shown. Each of the tubes 15 extends between and closely adjacent to apair of tubes 29 and both sets of tubes are mountedflush with the wall 8. This arrangement of the air and water tubes is advantageous but other arrangements may be employed. For example, one set of tubes may extend across the other set inheat exchange relation thereto and thereby maintain within safe limits the maximum temperatures' -attained by the air tubes during the operation of thefurnace.

Operation In operation, as fuel is consumed in the combustion chamber M, temperatures ranging from 1700 to 2000 degrees F. may be reached at the inner surface of the wall 8 upon which the tubes 15 and 29 extend. The tubes 29 are maintained at a much lower temperature by thewater flowing therein. The blowers. [8 are preferably of the constant pressure type. Air at room temperature is drawn into the blowers l8 and is forced at high velocity through the branches is, conduit 20 and header l6 and thence through the several tubes (5. In passing through these tubes the air is heated to the desired high temperature and then flows from the header 11 through the conduits 23, pipes 24 and'25 to the header 26 from which it is discharged at high velocity and temperature into the combustion chamber. As indicated in Fig. 4, the hot air is preferably injected and distributed uniformly through the body 12 of fuel where it enters the combustion chamber through the gate. This is particularly advantageous where the fuel has a high moisture content and is difficult to ignite, as in the case of sub-bituminous fuels, including lignite. My apparatus affords means for injecting air at temperatures of from 600 to 800 degrees F. into a body of such fuel so that the moisture content is rapidly reduced and the fuel is caused to ignite readily and to be consumed efficiently in the furnace.

The temperatures attained by the portions of the tubes I exposed to direct radiation within the combustion chamber are limited not only by the air passing at high velocity through these tubes but also by the cooling effect of the adjacent water walltubes 29. This has been found to be advantageous as a safeguard against rapid deterioration of the air tubes where they are exposed to the intense heat of direct furnace radiation.

Control of the temperature and pressure of the air delivered through the conduits 23 is afforded by my arrangement of the by-pass conduit 2! and dampers in this and the conduit 20 and branches 19. By suitable manipulation of the damper 22 in the conduit 2!, a controlled amountof the air at substantially room temperature may be bypassed into the header I! when it is desirable to deliver air at a temperature below the maximum. The cool, by-passed air merely mixes with the high temperature air in the header 5?. The elocity of flow through the heating tubes (5 may also be regulated by manipulation of the dampers in'the conduit 20 and branches [9 extending to the blowers. By adding high temperature proheated air to that required for combustion, increased efiiciency results even where preheating and drying'of the fuel is not required. In either case, improved combustion results from supplying air at temperatures ranging from 600 to 1000 degrees 'F. or at somewhat lower temperatures where this is desirable.

It will be evident that high temperature air supplied by my apparatus may be utilized for various purposes other than those specifically described herein, without departing from the spirit of my invention as defined in the appended claims.

Having described my invention, what I claim as new and desire to protect by Letters Patent is:

1. In a furnace adapted to burn sub-bitumi nous fuel and having a traveling grate for feeding such fuel into the combustion chamber and spaced parallel water wall tubes exposed to direct radiation from the source of heat in said chamber, an air preheater comprising, a plurality of metal tubes each extending along, in the space between and in heat exchange relation to adjacent water wall tubes and exposed to direct radiation from the flame within the furnace combustion chamber and at an elevation above said grate, means for supplying air at room temperature and at pressure above atmospheric to said tubes, said tubes being formed and arranged to heat the air discharged therefrom to temperatures above 600 degrees F., a conduit communicating with said tubes to receive the air heated therein and hot air discharge nozzles arranged to receive air from said conduit and to discharge the air directly into and below the upper surface of the body of fuel entering the combustion chamber on said grate.

2. In a steam generating furnace having a combustion chamber and spaced parallel water wall tubes exposed to direct radiation from the source of heat within said chamber, a multiplicity of metallic air tubes each extending along, in the space between and in heat exchange relation to adjacent water wall tubes within the combustion chamber and exposed to direct radiation from the source of heat therein, a header communicating with the several air tubes at one end thereof, a blower for supplying relatively cool air to said header and a conduit arranged to receive heated air from said tubes, said water wall tubes limiting by heat exchange the temperature of the air tubes within the combustion chamber.

3. In a steam generating furnace having a combustion chamber and water wall tubes exposed to direct radiation from the source of heat within said chamber, a multiplicity of metallic air tubes disposed in heat exchange relation'to said water wall tubes within the combustion chamber and exposed to direct radiation from the source of heat therein, a header communicating with the several air tubes at one end thereof, a blower for supplying relatively cool air to said header, a conduit arranged to receive heated air from said tubes, said water wall tubes limiting by heat exchange the temperature of the air tubes within the combustion chamber, a by-pass for cool air extending from said blower to said conduit and dampers adapted to selectively restrict the flow of air through said by-pass and to said air tubes whereby the temperature of the air delivered from said conduit may be controlled and limited by additions of controlled quantities of cool air from said by-pass.

CHARLES A. LARSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 430,726 Baxter June 24, 1890 841,944 Dollfuss Jan. 22, 1907 1,550,873 Brodbeck Aug. 25, 1925 1,683,804 Reed Sept. 11, 1928 1,951,971 Farbridge Mar. 20, 1934 2,119,817 Keller June '7, 1943 2,409,801 Ruegg Oct. 22, 1946 2,423,997 Ruegg July 15, 1947 FOREIGN PATENTS Number Country Date 530,636 Germany July 30, 1931 393,378 Great Britain June 8, 1933 

